Multifunctional photocatalytic paint coat and method of preparation thereof

ABSTRACT

The multifunctional paint is based on a highly porous inorganic substance created by a reaction of at least two components. TiO 2  nanoparticles are attached to the surface of this substance. The first component is a water insoluble calcium compound and the second component is a water soluble sulfate. The method of applying the multifunctional paint on the surface is that the first component containing the water suspension of the insoluble calcium compound is applied on the treated area first and subsequently a mixture of TiO 2  nanoparticles suspended in the water solution of the second component is applied over the first layer. Another way is to apply a water suspension of the first component containing also TiO 2  nanoparticles on the treated surface and then deposit the water solution of the second component on the layer. A mixture of all components can be also applied on the treated area at once.

TECHNICAL FIELD

The present invention relates to the multifunctional paints based onTiO₂ nanoparticles with photocatalytic and sanitary effects and the wayof applying the paint on walls, painted surfaces and constructionmaterials. The multifunctional coatings possess the covering, cleaning,sanitary, antibacterial, antimold, photocatalytic and otherfunctionalities.

BACKGROUND OF THE INVENTION

The usage of modern plastics and air-recycling systems in the buildingsrequire eliminating of the associated side-effects. Namely it is theslow release of undesirable aldehydes and roughly two hundred of otherorganic substances; and infections spreading through the HVAC systemsinto the whole building.

The fundamental functionalities of traditional paints and pigmentsalways lie in the color and hiding power. However, there are anincreasing number of attempts to add other functionalities to paints.From the historical point of view, the first such paint was lime, which,in the fresh form, functions as a white pigment and as a sanitary paintas well. Applications of the fundamental photocatalytic effect of TiO₂nanoparticles are very popular in the last decade. Although theeffectiveness of the photocatalytic effect of TiO₂ was examined verywell, all problems with the application of TiO₂ in photoactive paintshave not been satisfyingly solved yet.

If TiO₂ nanoparticles are mixed into the inorganic silicate paints, thesurface of nanoparticles is typically encapsulated by silicon oxide(SiO₂) and the desiderative photocatalysis is blocked. This surfacemodification of TiO₂ pigment particles by SiO₂ is used in the pigmentindustry for almost a century to decrease the photocatalytic effectpreventing the paint from chalking. The photocatalytic effect of thesilicate based products is in the best case residual and it amounts toonly percents or tenths of percent of the pure TiO₂ surfacephotocatalytic activity. TiO₂ nanoparticles are usually used to improvethe rheology of the paint rather than for its photocatalytic effect inthese types of composition.

The second most frequent practice of incorporating TiO₂ nanoparticlesinto the paint composition is to mix them directly into the acrylatebased paints. The problem of such solution is in the photocatalyticaggresivity of TiO₂ nanoparticles, which upon illuminationphotochemically break down and literally burn the surrounding acrylate.The result is strong chalking and yellowing of the paint.

Utilization of TiO₂ nanoparticles in the silicone based paints alsobrings serious limitations. Silicones, similar to silicates, effectivelyblock the TiO₂ surface eliminating the photocatalysis.

The application of sol-gel made TiO₂ nanoparticles directly on a surfaceis very costly and rather uncommon. The thickness of sol-gel TiO₂ spraycoating is around 50 nm, Nanoparticles of TiO₂ stick to the wallelectrostatically. The limitations of this technology are in the lowpurity of sol-gel TiO₂ nanocrystals and high acidity of the typicalsol-gel precursors. Another problem is the low TiO₂ quantity in the thincoating that uses only a minimal fraction of the available light.

In the so far known paints based on the photocatalytic effect of TiO₂nanoparticles, the photocatalysis is diminished because nanoparticlesget surrounded by a substance contained in the paint composition,typically the binder. This slows the photocatalytic degradation oforganic substances, especially cigarette smoke, some thinners,exhalations and aldehydes vaporized from the plastics on the paintedsurface.

SUMMARY OF THE INVENTION

The present invention of the multifunctional paints based on TiO₂nanoparticles with photocatalytic and sanitary effects provides asolution to the above mentioned shortages.

According to the invention the paint consists of a highly porousinorganic substance which is created by a chemical reaction of at leasttwo components. The TiO₂ nanoparticles are mounted onto the surface ofthe porous substance.

The first component is a water insoluble compound of calcium and thesecond component is a water soluble sulfate.

The insoluble calcium compound is preferably selected from the group ofcalcium carbonate, if possible with particle size in the nanometer range(further nano-calcium carbonate), lime or their mixtures.

The water soluble sulfate is preferably selected from the group ofcopper sulfate, zinc sulfate, silver sulfate or mixtures thereof.

The multifunctional paint preferably contains 5 to 90 wt % of TiO₂ inthe solids creating a photocatalytic layer in the range from 0.1 to 100micrometers in thickness.

The weight ratio of the second component (sulfate) to the firstcomponent (insoluble compound of calcium) is from 0.1:1 to 10:1.

The multifunctional paint favorably contains 3 to 80 wt % ofnano-calcium carbonate in the solids.

In one embodiment of the invention, the method of application of themultifunctional coating is that the first component consisting of thewater suspension of the insoluble calcium carbonate compound isdeposited on the treated area. After that, the suspension of TiO₂nanoparticles in a water solution of the second component (sulfate) isapplied on the top of this coat.

In another embodiment of the invention, it is possible to apply allcomponents simultaneously.

According to the invention, it is also practical to apply a coat of thecomponent consisting of the water suspension of the insoluble compoundof calcium with the suspended TiO₂ nanoparticles on the treated surfaceat first. In the following step a water solution of the second component(sulfate) is applied on the top of the first layer.

If the multifunctional paint is used to cover areas rich in insolublecompounds of calcium, namely stuccoed surfaces and concrete, it ispossible to treat the areas only with the suspension of TiO₂nanoparticles in a water solution of the second component (sulfate). Thesecond component reacts together with the insoluble compounds of calciumcreating a porous inorganic substance on the surface. The TiO₂nanoparticles are mounted onto the surface of the formed poroussubstance.

The paint can be conveniently manufactured and applied at the roomtemperature, typically 10 to 50° C.

The multifunctional paint is preferably applied on the ceiling, becausethe air circulation is the most effective there. To increase theefficiency of photocatalytic odor removal, especially from smoking andcooking, it is possible to increase the air turbulence by mounting a fanon the ceiling and illuminating the ceiling with a UV lamp.

Cleaning of the paint coat and refreshing of its functionality isrealized by an occasional illuminating of the painted area with anintensive source of UV light.

According to the invention, during the coating process a porousinorganic substance functioning as a binding agent is created. Theporous substance (binder) does not block the photocatalytic activity ofTiO₂ nanoparticles. Next to the binding inorganic structure, otheractive substances are created simultaneously during the reaction. Thesesubstances are present in the form of inorganic nanoparticles, whichaccording to the choice of reacting agents enable specific functions ofthe paint.

The paint can be applied on the surface in one-step using a reactingmixture, where all components are present in the suspension.

The paint can also be applied in two or three steps, where in theinitial step the first component of the reacting mixture is deposited onthe treated surface and TiO₂ with the other components are applied onthe top of it.

It is also possible to deposit TiO₂ with the first reactive component onthe surface initially and then spray or paint the other reactivecomponents on it.

It is convenient using calcium carbonate nanoparticles—CaCO₃ as thefirst component (FIG. 1B). Calcium carbonate nanoparticles are to thecertain extent replaceable with coarser CaCO₃, lime—CaO or better withcalcium hydroxide—Ca(OH)₂.

Water soluble sulfates are used as a second component, primarily CuSO₄,Ag₂SO₄ and ZnSO₄ which in the chemical reaction with nano-calciumcarbonate give a porous, acicular, fibrous or laminar structureconsisting of gypsum (CaSO₄.2H₂O) and freshly formed activenanoparticles of metal oxides and hydrated metal oxides with highlyporous surface and very strong antibacterial properties.

FIGS. 2 A, B and C show the morphology of materials created by thereaction of the sulfates with the nano-calcium carbonate. Although thechemical composition of paints created by this method is not exactlydefined, their preparation is consistently reproducible and the functionof each component is obvious from the testing.

Based on antibacterial and photocatalytic tests performed with thematerials, it is possible to assume that the function of nanoparticlescreated by the reaction of nano-calcium carbonate with zinc sulfate isin the ability to create strong antibacterial environment, which forinstance additionally prevents gangrene and yeast from growth andspreading. The functionality of the surface is stronger in the presenceof light. This disproportion can be caused by the photocatalyticproperty of ZnO.

The photocatalytic effect of TiO₂ nanoparticles mounted onto the surfaceof the porous substance created by the reaction of the insoluble calciumcompounds with the sulfates causes decomposition of organic substanceson the TiO₂ surface providing the paint antibacterial property and theability to remove odors.

The functionality of nanoparticles produced by the reaction of coppersulfate with nano-calcium carbonate, is the prevention of mold fromspreading and the antibacterial effect. The presence of copper basednanoparticles in the paint is desirable in locations where the moldprevention is necessary.

The functionality of nanoparticles created by the reaction of silversulfate with nano-calcium carbonate is again antibacterial. The paintcontaining these nanoparticles can be conveniently used in poorlylighted places and at night, when the antibacterial activity of thephotocatalytic TiO₂ is not sufficient.

According to the invention, it is possible to modify the paintproperties to certain extent by changing the composition, or theconcentration ratio of the components. It does not matter whether thepaint is created “in situ” in several steps, or by mixing all componentstogether before they are applied on a surface.

In order to ensure the functionalities mentioned above it is fundamentalcreating the porous structure partially consisting of gypsum. Gypsum isa side product of the sulfate reaction with the nano-calcium carbonate.Gypsum crystals bind the paint layer together, while they do not envelopclusters of nanoparticles of TiO₂ and the other active substances.

FIG. 2 D shows the structure composed of large gypsum crystals with thezinc based clusters of nanoparticles and TiO₂ incorporated onto it. Thestructure was created by the reaction of the components at the waterboiling point. The temperature at which these paints are manufacturedplays an important role. Generally, the range from 10° C. to 50° C. isthe most convenient for both manufacturing and application of thepaints. At lower temperature, a finer structure is created, but thereaction of the components is slower.

The most advantageous composition of the paint contains a sufficientamount of reacting substances, first to ensure the requiredfunctionality and second to create the porous gypsum based bindingstructure. The content of nano-calcium carbonate in the paint,potentially substitutable by calcium hydroxide, should be more than 3and less than 80 wt % of the solids. The optimum ratio of nano-calciumcarbonate in the paint is from 20 to 50 wt % of the solids.

The amount of the sulfate based compounds (second component) to thefirst component (nano-calcium carbonate) should be in the in the ratiofrom 0.1:1 to 10:1, depending on the character of the treated surface.

For proper odor removal and photodegradation of the organic compoundsthe amount of TiO₂ in the paint should be 10-90 weight percent of thesolids, optimally 50 to 80 weight percent.

The thickness of the multifunctional paint coat necessary for therequired functionality varies from 0.1 to 100 micrometers. To ensure thephotocatalytic function of paint, thickness of the layer should be inthe range from 1 to 10 micrometers, optimally 2 to 5 micrometers.

The invention describes the appearance and composition of themultifunctional paints. It also resolves the deposition of the cleaningand sanitary multifunctional paints on the surface. These paints arebased on the combination of TiO₂ nanoparticles and reacting components,which, after they are mixed together, provide a highly porous inorganicsubstance with TiO₂ nanoparticles mounted onto its surface withoutblocking of their photocatalytic effect. In addition, the reactingcomponents create other active compounds during the reaction. Thesenewly formed compounds give the paint other desirable properties, namelyantiviral and antibacterial effects, photocatalytic potency to decomposeorganic substances cleaning the air, eventually preventing mold andgangrene from spreading.

The effectiveness of the paint can be enhanced several times byincreasing the air circulation and intensity of the light. For examplethis can be done by mounting a fan and a “black light” fluorescent lampon the ceiling. Cleaning and restoring of the paint is performed byillumination of the painted area by an intense UV light.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows scanning electron microscope (SEM) micrographs of:

-   -   A) TiO₂ nanoparticles—the non-reactive component of the paint        ensuring the photocatalytic functionality    -   B) CaCO₃ nanoparticles—the reactive component of the paint

FIG. 2 shows SEM micrographs of:

-   -   A) Nanoparticles of the copper based active component, created        by the reaction of nano-calcium carbonate with copper sulfate        (CaCO₃ with CuSO₄).    -   B) Nanoparticles of the silver based active component, created        by the reaction of nano-calcium carbonate with silver sulfate        (CaCO₃ with AgSO₄).    -   C) Nanoparticles of the zinc based active component, created by        the reaction of nano-calcium carbonate with zinc sulfate (CaCO₃        with ZnSO₄).    -   D) The mixture of TiO₂ nanoparticles and the zinc based active        component, created by the reaction with nano-calcium carbonate        (CaCO₃) are incorporated onto the porous structure of gypsum,        which was also created in the reaction of nano-calcium carbonate        (CaCO₃) with ZnSO₄ at elevated temperature (100° C.).

FIG. 3 shows a SEM micrograph of the multifunctional paint, containingTiO₂ nanoparticles incorporated into the structure created by thereaction of nano-calcium carbonate with 2.5 wt % ZnSO₄ water solution.

EXAMPLES

The following examples illustrate, but do not limit, the presentinvention.

Example 1

As a first step, water suspension of nano-calcium carbonate—CaCO₃ andcalcium hydrate—Ca(OH)₂ mixture in the weight ratio 9:1 was applied onthe wall which was previously painted with an acrylic paint. Thesuspension contained 20 wt % of both components. This layer created thereactive base for the second component of the composite multifunctionalpaint. After the first layer dried, a suspension containing 7 wt % ofTiO₂ nanoparticles in the 2.5 wt % ZnSO₄ solution was sprayed on thetreated surface as a second step. Practically inert TiO₂ doesn'tparticipate in any reaction and it is consequently mounted onto thefibrous structure created by the reaction of the nano-calcium carbonateand calcium hydroxide mixture with ZnSO₄. The created paint coat isshown in the picture 3. Although the exact composition of the createdpaint is not exactly known, it is possible to assume that the paintconsists of clusters of TiO₂ nanoparticles, which are mechanicallymounted onto the structure consisting of nanoparticles of zinc oxide,hydrated zinc oxide, zinc carbonate, partially unreacted CaCO₃ and CaO,and gypsum—CaSO₄.2H₂O.

This 5 micrometers thick paint coat, in the daylight, exhibits fourtimes faster cleaning of the air contaminated by the smoke from onecigarette compared to the reference room. The cigarette smell vanishescompletely from the room painted with the multifunctional paint after 20minutes, leaving no musty smell in contrast to the reference room. Theantibacterial functionality of this paint coat shows an instant killingof all bacteria contamination, contrary to the reference acrylic paint,where the bacteria E. Coli did not completely die even after four hours.

Example 2

As a first step, a water suspension of CaCO₃ and TiO₂ nanoparticles inthe weight ratio 1:1 was applied on the stuccoed surface. The suspensioncontained 10 wt % of both substances. After this layer dried, it createdthe reactive base for the second component of the compositemultifunctional paint. As a second step 5 wt % ZnSO₄ solution wassprayed on the surface. Similar to the first example, after the paintdried, the inert TiO₂ was mounted onto the porous, fibrous structurecreated by the reaction of nano-calcium carbonate with zinc sulfate. Inthe daylight, the paint coat shows a similar ability to remove odors asin the example 1.

Example 3

Ten liters of a solution containing 0.9 kg of zinc sulfate and 5 gramsof silver sulfate was added into 10 liters of a cold water suspensioncontaining 1 kg of nano-calcium carbonate and 1 kg of TiO₂ nanoparticlesand mixed intensively. After few hours this slowly reacting mixture wasapplied with the roller on walls previously painted with an acrylicpaint. After the paint coat completely dried, a porous photocatalyticlayer about 10 micrometers thick was created. It had a strongantibacterial functionality even without the presence of light.

Example 4

One liter of a cold water suspension containing 0.07 kg of nano-calciumcarbonate and 0.1 kg of TiO₂ nanoparticles was prepared first. In aseparate container, active components were prepared in one liter ofwater suspension by the reaction of 0.1 kg CuSO₄ and 5 grams of AgNO₃with 0.2 kg of nano-calcium carbonate. Both suspensions were blendedtogether and properly mixed. After that, one liter of water solutioncontaining 0.1 kg of zinc sulfate was mixed in. The created mixture wasslightly diluted with water and applied on the surface with a brush.After the paint completely dried, the result was a porous layer of themultifunctional paint about 50 micrometers of thickness.

Example 5

A suspension containing 7 wt % of TiO₂ nanoparticles in 10 wt % watersolution of ZnSO₄ was sprayed on a concrete surface. After drying theresult was a paint coat with the TiO₂ nanoparticles incorporated ontothe porous and fibrous structure created by the reaction of the concretecalcium components with ZnSO₄. The created layer shows very goodphotocatalytic activity, it destroys lichen, algae and othermicroorganisms, which cause erosion of concrete. The paint maintains theconcrete surface free from these organisms for at least one year.

INDUSTRIAL UTILIZATION

The multifunctional paints are usable as sanitary paint coats inhospitals, biolabs, offices and residential buildings, namely in roomsfor allergics. They remove repulsive odors from rooms and work places,for example in restaurants. These painted surfaces are also suitable forair cleaning from the automobile exhalations, for example for facades ofbuildings, concrete sound barriers and so on. They are convenientlyapplicable in the livestock production to lower the risk of infectiousdiseases of the particular breed.

The invention claimed is:
 1. A multifunctional paint coat with thephotocatalytic and sanitary effects, based on TiO₂ nanoparticles,wherein TiO₂ nanoparticles are mounted onto nanoparticles of a highlyporous inorganic substance, wherein the highly porous inorganicsubstance is a reaction product of two components, wherein the firstcomponent is at least one water insoluble compound of calcium selectedfrom the group of nano-calcium carbonate, lime, calcium hydroxide andmixtures thereof and the second component is at least one water solublemetal sulfate selected from the group of copper sulfate, zinc sulfate,silver sulfate or their mixtures, and wherein the content of TiO₂ isfrom 5 to 90 weight percent, and the thickness of the photocatalyticpaint coat is in the range from 0.1 to 100 micrometers.
 2. Themultifunctional paint coat according to claim 1, wherein the weightratio of the second component (sulfate) to the first component(insoluble calcium compound) is from 0.1:1 to 10:1.